These days most instrumentalists are required to play all sorts of different effects on their instruments. For most of its life the horn has been required to play one note at a time, but it's actually possible to play more than one note. These combinations are called 'multiphonics' and, by and large, are only found in music composed in the last 40 or 50 years.
They are not that common but they are a good indication of the ever greater range and variety of things a modern horn-player is being asked to play.
Steve Bell has a few words to say about the 'multiphonics' he has played.